Technique for routing data within an optical network

ABSTRACT

A technique for routing data within an optical network having a plurality of network nodes is disclosed. In one embodiment, the technique is realized by receiving data at a first network node via a first optical signal having a first wavelength. The first wavelength corresponds to a first optical frequency, and the first optical frequency is mapped to a first binary representation. The first binary representation is divided into a first plurality of fields, wherein at least one of the first plurality of fields corresponds to a routing label in a first label stack. A top routing label in the first label stack indicates a second network node. Based at least partially upon the top routing label, the data is transmitted from the first network node to the second network node via a second optical signal having a second wavelength. The first wavelength may be either the same as or different from the second wavelength.

FIELD OF THE INVENTION

The present invention relates generally to optical wavelength-switchingand optical burst-switching and, more particularly, to a label-switchingtechnique for routing data within an optical network.

BACKGROUND OF THE INVENTION

Emerging metropolitan optical networks are based on opticalcross-connects (OXCs) controlled by internet protocols (IPs). Bothelements perform the critical task of optical bandwidth provisioning.These networks currently provide end-to-end circuit-switched bandwidthallocation, through routing protocols like Open Shortest Path First(OSPF), and through signaling protocols like Multi-Protocol LabelSwitching (MPLS). The increasing needs for greater network connectivity,and efficient bandwidth utilization require more dynamic bandwidthprovisioning schemes such as optical burst-switching (OBS) (see C. Qiao,and M. Yoo, “Choices, features and issues in optical burst-switching”,SPIE Optical Networks Magazine, vol. 1, pages 36–44, April 2000).However, this potential evolution does not change the need forhigh-quality wavelength-switched optical service in the near to mediumterm. In this context, it is appropriate to design optical switchingparadigms capable of efficiently supporting both types of services.However, the design of a common switch platform for wavelength-switchingand optical burst-switching (OBS) raises problems, because of theirquite different provisioning time-scales. In wavelength-switching,lightpaths are provisioned for hours, days or even months, while inoptical burst-switching (OBS) bandwidth is reserved on burst timescales, which are measured in sub-milliseconds units. State of the artoptical cross-connects (OXCs) built with micro-electro-mechanicalsystems (MEMS) technology may have cross-connection latencies expressedin tens of milliseconds (see N. R. Jankowski, C. Bobcowski, D. Zipkin,R. R. Krchnavek, and R. Chamberlain, “MEMS-based optical switch designfor reconfigurable fault-tolerant optical backplanes”, Proceedings ofthe 6^(th) International Conference on Parallel Interconnects, pages149–156, October 1999). These devices certainly have enough agility tosupport wavelength-switching, but cannot establish opticalcross-connections on a per-burst basis as it is required by opticalburst-switching (OBS).

In view of the foregoing, it would be desirable to provide opticalwavelength-switching and optical burst-switching techniques for routingdata within an optical network which overcome the above-describedinadequacies and shortcomings.

SUMMARY OF THE INVENTION

According to the present invention, a technique for routing data withinan optical network having a plurality of network nodes is provided. Inone embodiment, the technique is realized by receiving data at a firstnetwork node via a first optical signal having a first wavelength. Thefirst wavelength corresponds to a first optical frequency, and the firstoptical frequency is mapped to a first binary representation. The firstbinary representation is divided into a first plurality of fields,wherein at least one of the first plurality of fields corresponds to arouting label in a first label stack. A top routing label in the firstlabel stack indicates a second network node. Based at least partiallyupon the top routing label, the data is transmitted from the firstnetwork node to the second network node via a second optical signalhaving a second wavelength. The first wavelength may be either the sameas or different from the second wavelength.

In accordance with other aspects of the present invention, the toprouting label may be popped off the first label stack so as to promote anext routing label in the first label stack. If such is the case, thesecond wavelength corresponds to a second optical frequency, wherein thesecond optical frequency is mapped to a second binary representation.The second binary representation is divided into a second plurality offields, wherein at least one of the second plurality of fieldscorresponds to a routing label in a second label stack. The top routinglabel in the second label stack indicates a third network node. Also,the top routing label in the second label stack corresponds to the nextrouting label in the first label stack. Further, if the networkaccommodates 2^(N) frequencies in the form of,f _(i) =f ₀ +i·Δfwherein i=0, 1, . . . 2^(N)−1, the second optical frequency may bedefined by,f _(i) _(out) =f ₀+2^(L)((f _(i) _(in) −f ₀)−2^(N−L) .l.Δf)and,i _(out)=2^(L)(i _(in)−2^(N−L) .l)wherein fi_(in) represents the first optical frequency, l represents thevalue of the top routing label in the first label stack, and Lrepresents the bit length of the top routing label in the first labelstack.

In accordance with further aspects of the present invention, the toprouting label in the first label stack may be swapped with a new routinglabel when the first label stack contains more than two routing labels.If such is the case, the second wavelength corresponds to a secondoptical frequency, wherein the second optical frequency is mapped to asecond binary representation. The second binary representation isdivided into a second plurality of fields, wherein at least one of thesecond plurality of fields corresponds to a routing label in a secondlabel stack. A top routing label in the second label stack indicates athird network node. Also, the top routing label in the second labelstack corresponds to the new routing label. Further, if the networkaccommodates 2^(N) frequencies in the form of,f _(i) =f ₀ +i·Δfwherein i=0, 1, . . . 2^(N)−1, the second optical frequency may bedefined by,f _(i) _(out) =f _(i) _(in) +2^(N−L)(l−l ¹)Δfand,i _(out) =i _(in)+2^(N−L)(l−l ¹)wherein fi_(in) represents the first optical frequency, l¹ representsthe value of the top routing label in the first label stack, lrepresents the value of the new routing label, and L represents the bitlength of the top routing label in the first label stack.

In accordance with still further aspects of the present invention, a newrouting label may be pushed onto the first label stack. If such is thecase, the second wavelength corresponds to a second optical frequency,wherein the second optical frequency is mapped to a second binaryrepresentation. The second binary representation is divided into asecond plurality of fields, wherein at least one of the second pluralityof fields corresponds to a routing label in a second label stack. A toprouting label in the second label stack indicates a third network node.Also, the top routing label in the second label stack corresponds to thenew routing label. Further, if the network accommodates 2^(N)frequencies in the form of,f _(i) =f ₀ +i·Δfwherein i=0, 1, . . . 2^(N)−1, the second optical frequency is definedby,

$f_{i_{out}} = {f_{0} + {{\lfloor \frac{( {f_{i_{in}} - f_{0}} )}{\Delta\; f} \rfloor \cdot 2^{- L} \cdot \Delta}\; f} + {{2^{N - L} \cdot l \cdot \Delta}\; f}}$and,

$i_{out} = {\lfloor \frac{i_{in}}{2^{L}} \rfloor + {2^{N - L} \cdot l}}$wherein fi_(in) represents the first optical frequency, l represents thevalue of the top routing label in the second label stack, and Lrepresents the bit length of the top routing label in the second labelstack.

In accordance with still further aspects of the present invention, atleast another one of the first plurality of fields may correspond to atermination field indicating an end of the first label stack. Also, atleast another one of the first plurality of fields may correspond to acontention field for use in differentiating the first wavelength from athird wavelength. If such is the case, the data is defined as a firstdata, and a second data is received at the first network node via athird optical signal having the third wavelength, wherein the firstoptical signal and the third optical signal have similar routing pathsthrough the network.

The present invention will now be described in more detail withreference to exemplary embodiments thereof as shown in the appendeddrawings. While the present invention is described below with referenceto preferred embodiments, it should be understood that the presentinvention is not limited thereto. Those of ordinary skill in the arthaving access to the teachings herein will recognize additionalimplementations, modifications, and embodiments, as well as other fieldsof use, which are within the scope of the present invention as disclosedand claimed herein, and with respect to which the present inventioncould be of significant utility.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to facilitate a fuller understanding of the present invention,reference is now made to the appended drawings. These drawings shouldnot be construed as limiting the present invention, but are intended tobe exemplary only.

FIG. 1 shows a conventional label stack in a data packet.

FIG. 2 shows a conventional label-swap operation.

FIG. 3 shows a conventional label-pop operation.

FIG. 4 shows a conventional label-push operation.

FIG. 5 shows a system wherein label-switched routing is performed usinga conventional label-swap operation.

FIG. 6 shows a system wherein label-switched routing is performed usinga conventional label-pop operation.

FIG. 7 shows a wavelength-switched WDM system wherein conventionalwavelength-switching is performed.

FIG. 8 illustrates the two-way reservation process that takes place inthe wavelength-switched WDM system of FIG. 7.

FIG. 9 shows a burst-switched WDM system for illustrating a conventionalburst-switched transmission.

FIG. 10 shows a label stack format in accordance with the presentinvention.

FIG. 11 shows a label-swap operation in accordance with the presentinvention.

FIG. 12 shows a label-pop operation in accordance with the presentinvention.

FIG. 13 shows a label-push operation in accordance with the presentinvention.

FIG. 14 shows an example of wavelength contention resolution inaccordance with the present invention.

FIG. 15 shows a wavelength-switched WDM system that employs a label-popoperation in accordance with the present invention.

FIG. 16 illustrates the two-way reservation process that takes placeprior to the establishment of a connection in the system of FIG. 15.

FIG. 17 shows a wavelength-switched WDM system that employs a label-swapoperation in accordance with the present invention.

FIG. 18 illustrates the two-way reservation process that takes placeprior to the establishment of a connection in the system of FIG. 17.

FIG. 19 shown a wavelength-switched WDM system that employs both thelabel-push operation and the label-pop operation to provide transparentwavelength-trunking between two metropolitan area networks.

FIG. 20 illustrates the two-way reservation process that takes placeprior to the establishment of a connection in the system of FIG. 19.

FIG. 21 shows an optical burst-switched WDM system that employs alabel-pop operation to successfully switch an optical burst inaccordance with the present invention.

FIG. 22 shows the optical burst-switched WDM system of FIG. 21 whereinan optical burst is unsuccessfully switched due to a wavelength alreadyhaving been assigned to an ongoing circuit.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENT(S)

Label-switching comprises of all the data routing and switchingparadigms where packets are forwarded to next-hop nodes according tospecific labels. In these schemes, there is no route computation on aper-packet basis, and there is often a one-to-one mapping between labelsand outgoing interfaces at network nodes. Label-switching enables thenetwork to replace computation intensive layer 3 operations (e.g. routecomputation) by simpler layer 2 operations, which are implemented withdedicated high performance hardware.

Label-switching relies on three fundamental operations, which arelabel-swap, label-pop, and label-push. To explain these operations, weassume that in each packet header, the labels are organized into astack, as shown in FIG. 1. That is, FIG. 1 shows a data packet 10comprising a payload 12 and a header 14, wherein the header 14 comprisesa label stack 16 including a plurality of labels 18.

The label-swap operation simply replaces the top label 18 a of the stack16 with a new label 20 contained in a switching table (not shown), asshown in FIG. 2. The binding between the old label 18 a and the newlabel 20 is usually defined at connection setup.

The label-pop operation simply pops the top label 18 a of the stack 16,as shown in FIG. 3.

The label-push operation simply inserts a new label 24 into the stack16, as shown in FIG. 4.

The joint use of the label-pop operation and the label-push operationenables the definition of virtual paths.

The definition of routing labels may be static or dynamic. In the staticcase, the labels and the mapping between labels and next-hop nodes arebased on configuration information, as in the case of MPLS hop-by-hoprouting. When labels are defined dynamically, the labels or the bindingbetween labels and next-hop nodes are set on a per-flow basis, by asignaling protocol.

FIG. 5 shows a system 30 wherein label-switched routing is performedusing the label-swap operation. That is, the system 30 comprises asource 32, a destination 34, and a network 36, which includes a firstedge node 38, a second edge node 40, and a plurality of routers 42interconnected by a plurality of interconnects 44. As a data packetadvances across the network 36 from the source 32 to the destination 34,the top label of the label stack is simply replaced with a new labelcontained in a switching table, as shown (e.g., at router D, the toplabel 11 from incoming router B is replaced with new top label 01 foroutgoing router E). As mentioned above, the binding between the oldlabel and the new label is usually defined at connection setup.

FIG. 6 shows a system 50 wherein label-switched routing is performedusing the label-pop operation. The system 50 is physically identical tothe system 30 and thus is numerically identified as such. However, inthe system 50, as a data packet advances across the network 36 from thesource 32 to the destination 34, the top label of the label stack issimply popped off the label stack, as shown (e.g., at router D, the toplabel 11, indicating router E, is popped off the label stack). Thisoperation reduces the header overhead of packets as they get closer toegress nodes.

The label-switching paradigm has also been extended to wavelengthdivision multiplexing (WDM) networks. In fact, IP label-switchingprotocols have been extended to support wavelength-switching, wherelightpaths are established and deleted according to a circuit-switchingparadigm. These extensions use wavelengths as labels, and have beencalled optical label-switching protocols (see N. Ghani,“Lambda-labeling: A framework for IP-over-WDM using MPLS”, SPIE OpticalNetworks Magazine, vol. 1, pages 45–58, April 2000). However, theseprotocols do not explicitly encode route information in wavelengths orinterfaces (i.e., it is impossible to predict the next hop of an opticalpacket by simply considering its incoming interface and its wavelength).Instead, the assignment of wavelengths to outgoing interfaces is assumedto be utterly arbitrary, and only decided at connection setup, on aper-flow basis. Although waveband routing appears as an option ofcurrent optical label-switching protocols, these protocols do notaddress the appropriate definition of wavebands to providesource-routing for optical burst-switching (OBS) or circuit-switchedwavelength routing.

Referring to FIG. 7, there is shown a wavelength-switched WDM system 60that may be operating under any of the protocols that have heretoforebeen proposed for optical label-switching. The wavelength-switched WDMsystem 60 comprises a source 62, a destination 64, and a network 66,which includes a first edge node 68, a second edge node 70, and aplurality of optical cross-connects (OXC) 72 interconnected by aplurality of interconnects 74. As shown in FIG. 7, as a data packetadvances across the network 66 from the source 62 to the destination 64,the wavelength at which the data packet is transmitted changes.

It is assumed that there is a request for a lightpath with a bandwidthof one wavelength between the source 62 and the destination 64. Theassignment of wavelengths to the different portions of the lightpath isonly based on the availability of the wavelength channels, and on thewavelength-conversion capabilities. Bindings between incoming andoutgoing channels are stored in connection tables, as shown in FIG. 7.The connection table entries corresponding to the lightpath requestbetween the source 62 and the destination 64 are represented by givingthe incoming node and wavelength, as well as the outgoing node andwavelength. For example, at a first optical cross-connect 72 (i.e., OXCB), the entry (A,F₁₁):(D,F₁₁) means that the channel coming from thefirst edge node 68 at optical frequency F₁₁ is to be switched to asecond optical cross-connect 72 (i.e., OXC node D) at frequency F₁₁.FIG. 8 illustrates the two-way reservation process that takes placeprior to the establishment of the connection between the source 62 andthe destination 64 in the system 60 of FIG. 7.

Optical burst-switching (OBS) is based on a tell-and-go transmissionmodel wherein traffic sources send control packets ahead of data burstsand do not wait for resource confirmation messages before transmittingdata packets (see C. Qiao, and M. Yoo, “Choices, features and issues inoptical burst-switching”, SPIE Optical Networks Magazine, vol. 1, pages36–44, April 2000). The burst transmission latency is the amount of timethat separates control packets and data bursts. It is computed atnetwork ingress nodes to give the network enough time to setup therequested circuits. In WDM networks, optical burst-switching (OBS)control packets request wavelength cross-connections on a per-burstbasis. Assuming that the OXCs in the network have a cross-connectionlatency (i.e., the time it takes to connect two physical ports in anOXC) of δ_(x), a message processing delay of δ_(m), and an H hop pathwith propagation delay of δ_(p), the burst transmission latency must beat least equal to δ_(min)=H.(δ_(x)+δ_(m))−δ_(p).

Referring to FIG. 9, there is shown a burst-switched WDM system 80 forillustrating a burst-switched transmission. The system 80 is physicallyidentical to the system 60 and thus is numerically identified as such.Also, similar to the system 60, in the system 80, as a data packetadvances across the network 66 from the source 62 to the destination 64,the wavelength at which the data packet is transmitted changes. However,in the system 80, the data packets are sent in bursts preceded bycontrol packets.

The efficiency of the burst-switching paradigm decreases as the bursttransmission latency increases. Unfortunately, state-of-the art OXCsstill have cross-connection latencies (δ_(x)) in the tens ofmilliseconds range, which is a dominant parameter of the overallburst-transmission latency and imposes an upper limit on attainablethroughputs.

To overcome the problems associated with the aforementioned conventionallabel-switching schemes, a new label-switching paradigm is disclosed.This new label-switching paradigm involves the explicit encoding ofrouting information through specific wavelength assignments. That is, inthis new label-switching paradigm, a wavelength is equivalent to abinary string. This binary string is partitioned into routing bits, andcontention resolution bits. The two types of bits are handled bydisjoint operations to meet the routing and the contention resolutionneeds of the network. Thus, this new label-switching paradigm isrealized via a new label-stack format, and the processing of opticallabels through specific label-stack operations, which are both describedin detail below.

At the outset, it is assumed that the new label-switching paradigm isoperating in a dense wavelength division multiplexing (DWDM) networkwith W=2^(N) optical frequencies of the form f_(i)=f₀+i.Δf, where i=0, .. . , 2^(N)−1. It is also assumed that this network has properwavelength-conversion facilities at all network nodes to support thedifferent operations described below.

Referring to FIG. 10, there is shown a label stack format 90 inaccordance with the present invention. To encode the label stack, eachoptical frequency, f_(i), is mapped to the binary representation of i.For example, when N=8, frequency f₁₆₁ is mapped to binary number10100001. The binary number of the optical frequency is then dividedinto smaller fields corresponding to routing labels. The obtained set oflabels (i.e., Label 1, Label 2, etc.), read from left to right, and anoptional stack termination string (i.e., End label), define the labelstack of the wavelength channel. The lengths of the routing labels andtheir interpretation may be node and interface dependent. Therefore, thelabel stack associated with a particular wavelength depends on theselected lightpath.

To illustrate the foregoing, first consider N=8, a fixed label lengthL=2, and a stack termination string equal to 00. Then, the label stackof frequency f₁₆₁ includes two labels that are l₁₆₁ ¹=10, l₁₆₁ ²=10.However, if the lightpath goes through two nodes supportinglabel-switched routing, such that the first node uses labels of lengthL₁=1, and the second node uses labels of length L₂=3, two differentlabels are obtained for f₁₆₁, which are l₁₆₁ ¹=1, l₁₆₁ ²=010.

The label-stack operations include label-swap, label-pop, and label-pushoperations. The label-swap operation replaces the first label of thestack with another label, to be used by the next-hop. This operationrequires that the swapped labels have the same length. The label-swapoperation can be mathematically defined as follows.

Consider some frequency f_(i) _(in) , such that the first label of thestack is l¹ of length L, and some other label 0≦l<2^(L). When l¹ isreplaced with l, the frequency f_(i) _(out) is obtained which satisfiesthe following equations:f _(i) _(out) =f _(i) _(in) +2^(N−L)(l−l ¹)Δfi _(out) =i _(in)+2^(N−L)(l−l ¹)FIG. 11 further illustrates the label-swap operation 100 wherein thefirst label (i.e., Label 1) in an initial label stack 102 is replacedwith a new label (i.e., New label) in a resulting label stack 104.

To give an example of the label-swap operation, consider, as before,N=8, a fixed label length L=2, and a stack termination string equal to00. The first label of the stack of frequency f₁₆₁ is l₁₆₁ ¹=10. Whenthis first label is replaced (i.e., swapped) with a new label l=01, theoutput frequency f_(i) _(out) =f₉₇ is obtained.

In the label-pop operation, labels that have already been used can beremoved from the label stack without any impact on the optical servicereceived by a wavelength channel. The removal of such useless labelsfrom the stack simplifies the task of subsequent network nodes. Toremove labels from the stack, a label-popping wavelength-conversion isperformed.

The precise mathematical definition of the label-pop operation for alabel, l, of length L (in number of bits) is given by the followingequations:f _(i) _(out) =f ₀+2^(L)((f _(i) _(in) −f ₀)−2^(N−L) .l.Δf)i _(out)=2^(L)(i _(in)−2^(N−L) .l)In this definition, it is assumed that l is the first label of the labelstack of f_(in). FIG. 12 further illustrates the label-pop operation 110wherein the first label (i.e., Label 1) in an initial label stack 112 isremoved (i.e., popped) in a resulting label stack 114.

To give an example of the label-pop operation, consider, as before, N=8,a fixed label length L=2, and a stack termination string equal to 00.When the frequency f₁₆₁ is popped, the output frequency f₁₃₂ isobtained. It is important to note that the label-pop operation alsoaffects the wavelength bits that are not part of the label stack.However, this operation can be defined in a way that does not affect thebits outside of the label stack.

The label-push operation is opposite to the label-pop operation as itintroduces new labels into the stack. The label-push operation is usefulto introduce new routing labels into the label stack at any point alongthe lightpath. However, pushed labels must be defined through priorsignaling or recovered from data packets by other means than wavelengthencoding.

The precise mathematical definition of the label-push operation for alabel l of length L (in number of bits) is given by the followingequations:

$f_{i_{out}} = {f_{0} + {{\lfloor \frac{( {f_{i_{in}} - f_{0}} )}{\Delta\; f} \rfloor \cdot 2^{- L} \cdot \Delta}\; f} + {{2^{N - L} \cdot l \cdot \Delta}\; f}}$

$i_{out} = {\lfloor \frac{i_{in}}{2^{L}} \rfloor + {2^{N - L} \cdot l}}$FIG. 13 further illustrates the label-push operation 120 wherein a newlabel (i.e., Label 0) in a resulting label stack 122 is introduced(i.e., pushed) over the first label (i.e., Label 1) in an initial labelstack 124.

To give an example of the label-push operation, consider, as before,N=8, a fixed label length L=2, and a stack termination string equal to00. When the label l=10 is pushed into frequency f₁₆₀, the outputfrequency f₁₆₈ is obtained.

In wavelength-switching networks, it is necessary to avoid wavelengthcontention. When such contention occurs, it can be resolved by changingthe wavelength bits that are not meaningful from the point of view ofrouting, which are hereafter referred to as wavelength contentionresolution bits. For example, consider, as before, N=8, a fixed labellength L=2, and a stack termination string equal to 00. Also, assumethat two wavelength-channels share the same network path starting fromsome common downstream node, and are both assigned the frequency f₁₆₁ inthe hop prior to a common node. To avoid collision on the next-hop link,one of these channels may be assigned to another frequency (e.g.,frequency f₁₆₃) before undergoing any label-stack operation. Of course,the newly assigned frequency (i.e., frequency f₁₆₃) must be available onthe link. It is easy to check that the wavelength-interchange thatoccurs does not modify the original routing information contained in thefrequency.

Referring to FIG. 14, a representation 130 of the above-describedwavelength contention resolution example is provided wherein datapackets are being transmitted from both a first OXC 132 (i.e., OXC A)and a second OXC 134 (i.e., OXC B) at the frequency f₁₆₁ to a third OXC136 (i.e., OXC C). The third OXC 136 assigns the transmission from thefirst OXC 132 to another frequency (e.g., frequency f₁₆₃) beforeundergoing any label-stack operation to avoid collision on the next-hoplink. In this particular example, the third OXC 136 assigns the newfrequency by changing the two least significant wavelength bits (i.e.,the wavelength contention resolution bits). The third OXC 136 thenperforms a label-pop operation on the incoming wavelength-channelsbefore forwarding them to a fourth OXC 138 (i.e., OXC D). In theabove-described example, it is assumed that the mapping between routinglabels and next-hop nodes is the same for all incoming interfaces.

The above-described new label-switching paradigm may supportwavelength-switching and wavelength-trunking, as well as opticalburst-switching. For all of these label-switching applications, the newlabel-switching paradigm offers many options to establish lightpaths,depending on the type of label-switching operation used at the differenthops of a path, and on the nature of the network hops. That is, in somecases each hop may map to a physical link, while in other cases somehops may be logical and correspond to virtual paths. In the followingdescription, examples of each of the above-mentioned label-switchingapplications are provided, although it should be noted that the presentinvention is not limited to these specific examples.

Referring to FIG. 15, there is shown a wavelength-switched WDM system150 that employs the label-pop operation in accordance with the presentinvention. The wavelength-switched WDM system 150 comprises a source152, a destination 154, and a network 156, which includes a first edgenode 158, a second edge node 160, and a plurality of opticalcross-connects (OXC) 162 interconnected by a plurality of interconnects164. In the wavelength-switched WDM system 150, a number W=256 (i.e.,N=8) of consecutive regularly spaced wavelength channels are assumed, asis a label length L=2. Also, it is assumed that the traffic demandcomprises one wavelength channel between the source 152 and thedestination 154.

The source 152 and the destination 154 access the core network throughthe first edge node 158 and the second edge node 160, respectively. Inthe core network, the plurality of optical cross-connects (OXC) 162provide waveband-routing by employing the label-pop operation inaccordance with the present invention. The mapping between next-hopnodes and labels is given for each node 162 in the core network by thesmall table located next to each node 162 in the core network, as shownin FIG. 15. For example, in the case of node OXC B, the mapping is thefollowing:

-   -   Next-hop A: 01    -   Next-hop C: 10    -   Next-hop D: 11

The correspondence between labels and wavebands is easily determined.For example, considering an arbitrary routing label b₁b₀, thecorresponding waveband is [b₁b₀000000,b₁b₀111111]. In other words, allfrequencies in the band [b₁b₀0000000,b₁b₀1111111] are sent to thenext-hop of the routing label b₁b₀, after being label-popped. Therefore,the mapping between next-hop nodes and routing labels can be specifiedby giving the mapping between next-hop nodes and wavebands. In the caseof node OXC B, the equivalent description is the following:

-   -   Next-hop A: [F₆₄, F₁₂₇]    -   Next-hop C: [F₁₂₈, F₁₉₁]    -   Next-hop D: [F₁₉₂, F₂₅₅]

After receiving a connection request from the source 152, the first edgenode 158 computes a path for the connection through the network 156.Assuming that this path is through OXC B, OXC D, OXC E, and OXC F, thefirst edge node 158 sends a path setup request along the selected path.This path setup request contains a list of wavelengths which may beassigned at each node 162 along the path, taking into account thefollowing constraints: 1.) At each hop, the assigned channel must beavailable; 2.) For each node 162, the outgoing assigned wavelengthchannel is obtained from the incoming wavelength-channel throughlabel-pop in the nodes 162 in the core network (i.e., it is notnecessary to pop labels when the next-hop is either the first edge node158 or the second edge node 160, although it is possible to do so); and3.) The path defined by assigned wavelength-channels must match the pathselected by the first edge node 158. FIG. 16 illustrates the two-wayreservation process that takes place prior to the establishment of theconnection between the source 152 and the destination 154 in the system150 of FIG. 15.

The result of this path setup process depends on the current congestionin the network 156. For simplicity, it is assumed that the result of thepath setup phase is the allocation of frequency F₂₅₅ between OXC A andOXC B, frequency F₂₅₂ between OXC B and OXC D, frequency F₂₄₀ betweenOXC D and OXC E, and frequency F₂₄₀ between OXC E and OXC F.

The label stack encoded in frequency F₂₅₅ can be checked to see if itmatches the selected path. For example, the binary representation ofF₂₅₅ is 11111111, which corresponds to four consecutive routing labelsthat are respectively l₁=11, l₂=11, l₃=11, and l₄=11. The first labell₁=11 is used at OXC B to select OXC D, as the label l₁=11 maps to OXC Dfor the next-hop. The application of the label-pop operation on F₂₅₅yields F₂₅₂. Similar checks may be done for the other nodes 162 of thepath.

Referring to FIG. 17, there is shown a wavelength-switched WDM system170 that employs the label-swap operation in accordance with the presentinvention. The system 170 of FIG. 17 is physically identical to thesystem 150 of FIG. 15 and thus is numerically identified as such.However, in the system 170 of FIG. 17, the wavelength-switched lightpathis formed by employing the label-swap operation, rather than thelabel-pop operation of the system 150 of FIG. 15.

In the system 170 of FIG. 17, the input wavelength does not carry thewhole route information. Instead, the binding between incoming andoutgoing routing labels is stored in connection tables of the nodes 162of the path at connection setup. At each hop of the path, the wavelengthof the lightpath is modified to carry the correct routing label for thenext hop through the label-swap operation, and to avoidwavelength-collision with other ongoing circuits. Such awavelength-interchange operation may be seen as a two-step processwherein the following two operations occur in sequence: 1.) Modificationof the wavelength bits carrying the routing labels; and 2.) Modificationof the wavelength contention resolution bits to avoid collision. The twooperations operate on disjoint sets of wavelength bits.

The mapping between next-hop nodes and labels is given for each node 162in the core network by the small table located next to each node 162 inthe core network, as shown in FIG. 17. Also shown next to each node 162in the core network is the connection table entry corresponding to theselected lightpath between the source 152 and the destination 154 whenthe node 162 belongs to the lightpath. As with the system 150, it iseasy to check that, at each hop of the lightpath, the routing label ofthe wavelength is swapped according to the target next hop node, andthat no wavelength-collision occurs. FIG. 18 illustrates the two-wayreservation process that takes place prior to the establishment of theconnection between the source 152 and the destination 154 in the system170 of FIG. 17.

In both of the above-described cases of wavelength-switched lightpaths(i.e., those utilizing the label-pop operation and the label-swapoperation), the route computation algorithm generally must include theconstraints corresponding to the mapping between wavebands and next-hopnodes which are imposed by the new label-switching paradigm inaccordance with the present invention. In both cases, the routeinformation is advertised at connection setup, and the wavelengthchannels must be explicitly allocated by network nodes.

In addition to the above-described cases of wavelength-switchedlightpaths (i.e., those utilizing the label-pop operation and thelabel-swap operation), wavelength-switched lightpaths may also berealized utilizing a combination of the label-push operation and thelabel-pop operation. This additional wavelength-switching application,termed wavelength-trunking, is particularly beneficial in local andmetropolitan area networks. That is, the costs of WDM systems increasewith the granularity of wavelength channels. In local and metropolitanarea networks, the costs of WDM nodes are still a determining factor ofthe commercial use of WDM. To address the cost issue, coarse WDM hasbeen proposed where channel spacings are much larger than in long-hauldense WDM systems. This difference in wavelength-channel granularitybetween the two types of networks reflect deeper functional differences.Local and metropolitan area networks put a stress on per wavelengthrouting and switching functions, while long-haul networks have a morepoint-to-point nature and perform per wavelength operations only at thetransmission layer to monitor the integrity of the optical signals.However, long-haul networks are evolving to perform a certain amount ofwavelength-routing and switching. The large numbers ofwavelength-channels, which may be soon supported by long-haul systems,prohibit the same level of functionality on a per wavelength-basis as inlocal or metropolitan networks. Therefore, waveband-switching schemesare appropriate to enable some wavelength-switching functionality inlong-haul networks, with the definition of wavelength trunkscorresponding to wavebands.

To implement such waveband-switching schemes, a combination of thelabel-push operation and the label-pop operation is used where a labelis pushed/popped at network ingress/egress nodes according to thewaveband or virtual path to which it is assigned. As in traditional datanetworks, virtual paths are defined by traffic engineering and trafficmanagement activities. An important future revenue generating service ofmetropolitan area networks is the definition of virtual private opticalnetworks, where different nodes are spread over different metropolitannetworks connected by long-haul trunks. For the implementation of suchservices, wavelength-trunking based on label-push/pop operations isparticularly appropriate because it is transparent to the nodes in thevirtual private network and it preserves other wavelength-trunkinginformation specific to the virtual private network.

Referring to FIG. 19, there is shown a wavelength-switched WDM system180 that employs both the label-push operation and the label-popoperation to provide transparent wavelength-trunking between twometropolitan area networks. The system 180 comprises a firstmetropolitan area network 182 and a second metropolitan area network 184that are both connected to a common long distance core network 186. Thefirst metropolitan area network 182 includes nodes A, B, and C, whilethe second metropolitan area network 184 includes nodes H, I, and J. Thecore network 186 includes a plurality of optical cross-connects (OXCs)188. Both the first metropolitan area network 182 and the secondmetropolitan area network 184 have a wavelength granularity that issixteen times coarser than that supported by the core network 186.

It should be assumed that all the networks 182, 184, and 186 share thesame frequency set F₀, F₁, . . . F₂₅₅. However, the frequencies used inthe first metropolitan area network 182 and the second metropolitan areanetwork 184 are of the form F_(16.i), where i is an integer between 0and 15. Also, the different networks may use any form ofwavelength-switching, including any of those described above (i.e.,those utilizing the label-pop operation and the label-swap operation).

In the core network 186, a virtual path (i.e., VP in FIG. 19) isestablished from OXC D to OXC G though OXC F. The virtual path carriesthe frequencies in the range [F₀, F₁₅] between OXC D and OXC F, and inthe range [F₆₄, F₇₉] between OXC F and OXC G. If a label length of 4 isconsidered in the core network 186, the consecutive values of thetopmost label of the virtual path are 0000 in the link between OXC D andOXC F, and 0100 in the link between OXC F and OXC G. For some connectionbetween a source node (S) 190 and a termination node (T) 192, a pathfrom node B to node C to OXC D to OXC F to OXC G to node H to node J isselected. The routing and wavelength assignment algorithm allocatesspecific frequencies in the links of the path included in the firstmetropolitan area network 182 and the second metropolitan area network184. Such assignment is based on the distribution of the load in thefirst metropolitan area network 182 and the second metropolitan areanetwork 184. To transport the connection over the core network 186, thevirtual path is used by selecting a frequency that meets the followingconditions: 1.) It belongs to the set used by the metropolitan areanetworks 182 and 184 (i.e., it is of the form F_(16.i)); 2.) It is freeover the node C to OXC D link and the OXC G to node H link; and 3.) Whenthe first label of the virtual path is pushed into the wavelength, awavelength is obtained that is available in the range allocated to thevirtual path. It is assumed that such a frequency is F₂₂₄, which becomesF₁₄ when label 0000 is pushed into the wavelength. In the virtual path,the allocated channel is F₁₄ between OXC D and OXC F, and F₇₈ betweenOXC F and OXC G. At OXC G, the allocated channel is label-popped (i.e.,the popped label is 0100) to obtain F₂₂₄. Overall, the operations in thecore network 186 are completely transparent to the first metropolitanarea network 182 and to the second metropolitan area network 184.

FIG. 20 illustrates the two-way reservation process that takes placeprior to the establishment of the connection between the source node (S)190 and the termination node (T) 192 in the system 180 of FIG. 19.

Referring to FIG. 21, there is shown an optical burst-switched WDMsystem 200 that employs the label-pop operation in accordance with thepresent invention. The optical burst-switched WDM system 200 comprises asource 202, a destination 204, and a network 206, which includes a firstedge node 208, a second edge node 210, and a plurality of opticalcross-connects (OXC) 212 interconnected by a plurality of interconnects214. Similar to the wavelength-switched WDM system 150 of FIG. 15, inthe optical burst-switched WDM system 200 of FIG. 21 a number W=256(i.e., N=8) of consecutive regularly spaced wavelength channels areassumed, as is a label length L=2. Also, it is assumed that the trafficdemand comprises one wavelength channel between the source 202 and thedestination 204. However, in the system 200 of FIG. 21, there is no needto establish physical cross-connections on a per-burst basis. That is,the system 200 of FIG. 21 may support optical burst-switching (OBS)provided that the network nodes 212 have the two following capabilities:1.) Interpretation of OBS control messages to resolve contention amongoptical bursts; and 2.) Assignment of absolute priority ofwavelength-switched traffic over burst-switched traffic. If such is thecase, the first edge node 208 may encode multi-hop route informationinto a burst by assigning it to the correct ingress wavelength. Innetworks of moderate size, all the route information may be encoded intothe wavelength and decoded at network nodes 212 by the label-popoperation, as shown in FIG. 21. In the example shown in FIG. 21, at eachnetwork hop (except the last one leading to the second edge node 210),the wavelength of the optical burst is popped according to the label-popoperation described above. Thus, the label-pop operation and thewaveband routing process provide high-performance burst forwarding byeliminating the need to establish physical cross-connections on aper-burst basis. Further, on an H hop path, the minimum bursttransmission latency reduces from H.(δ_(x)+δ_(m))−δ_(p) (which is thecase for conventional optical burst switching systems, as describedabove) to δ_(min)=H.δ_(m)−δ_(p).

At this point it should be noted that a proper contention resolutionmechanism is critical to successful transmissions. To assign absolutepriority to wavelength-switched lightpaths, a simple scheme isconsidered where optical bursts are dropped if they are carried onwavelengths already assigned to ongoing circuits, and where burstcollisions are allowed in the core network. In such a case, a burst issuccessfully transmitted when it does not use any busy wavelength on itspath, and does not collide with other bursts. As described above, FIG.21 illustrates a successfully transmitted burst. In contrast, FIG. 22illustrates an unsuccessful burst transmission in the system 200 of FIG.21 due to a wavelength already having been assigned to an ongoingcircuit.

As with the system 150 of FIG. 15 and the system 170 of FIG. 17, it iseasy to check that the wavelength carrying the optical burst in thesystem 200 of FIGS. 21 and 22 is changed according to the label-popoperation, and that the ingress wavelength carries the whole routeinformation. More sophisticated contention resolution schemes may beconsidered within the scope of the present invention where contendingoptical bursts are buffered or deflected.

As described above, the label-pop operation enables the efficientimplementation of source routing for optical burst-switching (OBS).However, labels may also correspond to virtual paths that are themselveslabel-switched. When the topmost label of an optical burst maps to sucha virtual path, the label stack of the virtual path may be pushed (i.e.,via the label-push operation) into the stack of the burst to enableproper forwarding. Thus, optical burst-switching (OBS) may also beefficiently implemented utilizing the label-push operation in accordancewith the present invention.

In view of the foregoing, the new label-switching paradigm describedherein supports wavelength-switching, wavelength-trunking, as well asoptical burst-switching, by encoding part or all of a network route intothe wavelength assigned to bursts or circuits at a network ingress node.The new label-switching approach is a new development in the area ofwaveband routing where, at network nodes, disjoint wavebands areassigned to outgoing interfaces, and incoming channels are routed tooutgoing interfaces according to their incoming interfaces and to theband to which they belong. Typically, the wavebands form a partition ofthe available spectrum. When optical frequencies are mapped tocorresponding binary numbers, the waveband of a particular opticalfrequency is identified by a specific bit pattern, which corresponds toa routing label.

Waveband routing is quite appropriate for optical burst-switching (OBS)because it does not require physical cross-connection on a per-burstbasis in optical cross-connects (OXCs). However, when using wavebandrouting, a major problem is the composition of the end-to-end routingservice, when the bands are defined locally. In the worst-case, thedifferent wavebands needed along specific paths may not intersect,therefore preventing the use of waveband routing for the selected paths.

An important difference between the present invention and previouswaveband routing schemes lies in a solution to the waveband-intersectionproblem. The solution is based on a systematic transformation of routinglabels at each network node to enable proper forwarding at the next-hop.These transformations use specific operations, which we call label-swap,label-pop, and label-push to be consistent with the MPLS model (see D.Awduche, “MPLS traffic engineering in IP networks”, IEEE Communicationsmagazine, vol. 37, pages 42–47, December 1999).

The label-swap operation involves replacing a current routing label byanother routing label that was previously advertised by the signalingprotocol at connection setup.

The label-pop operation occurs when the binary number corresponding to agiven optical frequency is partitioned into several consecutivebit-fields, which correspond to routing labels, and form a label stack.The label-pop operation involves popping the label stack by removing thetopmost label, and by shifting the position of all of the other labelsof the stack by a number of bits equal to the size of the removed label.The advantage of the label-pop operation is to enable source routing byencoding multiple routing labels when proper wavelengths are selected ata network ingress node.

The label-push operation inserts new routing labels into the stack. Incombination, label-pop and label-push operations also provide good meansto support wavelength-trunking, when network ingress nodes withclassifying functions push labels into wavelengths according to virtuallightpaths through which they must be routed.

All the above-mentioned label-stack operations only requirewavelength-conversion devices. For these reasons, they may beimplemented by all-optical means, which provide high data packetforwarding performance. The new label-switching paradigm decouples theoptical data forwarding process from the wavelength-contention problem.Therefore, network nodes are able to route optical packets or bursts ondemand, independent of quality-of-service (QoS) levels that result fromthe contention resolution process. Despite this added flexibility, thenew label-switching paradigm can still support traditionalwavelength-switching. Overall, the new label-switching paradigm enablesthe design of networks possessing mixed wavelength-switching and opticalburst-switching (OBS) capabilities supported on the same networkinfrastructure.

The present invention is not to be limited in scope by the specificembodiments described herein. Indeed, various modifications of thepresent invention, in addition to those described herein, will beapparent to those of ordinary skill in the art from the foregoingdescription and accompanying drawings. Thus, such modifications areintended to fall within the scope of the following appended claims.Further, although the present invention has been described herein in thecontext of a particular implementation in a particular environment for aparticular purpose, those of ordinary skill in the art will recognizethat its usefulness is not limited thereto and that the presentinvention can be beneficially implemented in any number of environmentsfor any number of purposes. Accordingly, the claims set forth belowshould be construed in view of the full breath and spirit of the presentinvention as disclosed herein.

1. A method for routing data within an optical network having aplurality of network nodes, the method comprising the steps of:receiving data at a first network node via a first optical signal havinga first wavelength, the first wavelength corresponding to a firstoptical frequency, the first optical frequency being mapped to a firstbinary representation, the first binary representation being dividedinto a first plurality of fields, at least one of the first plurality offields corresponding to a routing label in a first label stack, a toprouting label in the first label stack indicating a second network node;and based at least partially upon the top routing label, transmittingthe data from the first network node to the second network node via asecond optical signal having a second wavelength.
 2. The method asdefined in claim 1, further comprising the step of: popping the toprouting label off the first label stack so as to promote a next routinglabel in the first label stack.
 3. The method as defined in claim 2,wherein the second wavelength corresponds to a second optical frequency,the second optical frequency being mapped to a second binaryrepresentation, the second binary representation being divided into asecond plurality of fields, at least one of the second plurality offields corresponding to a routing label in a second label stack, a toprouting label in the second label stack indicating a third network node.4. The method as defined in claim 3, wherein the top routing label inthe second label stack corresponds to the next routing label in thefirst label stack.
 5. The method as defined in claim 4, wherein thenetwork accommodates 2^(N) frequencies in the form of,f _(i) =f ₀ +i·Δf wherein i=0, 1, . . . 2^(N)−1, wherein the secondoptical frequency is defined by,f _(i) _(out) =f ₀+2^(L)((f _(i) _(in) −f ₀)−2^(N−L) .l.Δf) and,i _(out)=2^(L)(i _(in)−2^(N−L) .l) wherein fi_(in) represents the firstoptical frequency, 1 represents the value of the top routing label inthe first label stack, and L represents the bit length of the toprouting label in the first label stack.
 6. The method as defined inclaim 1, further comprising the step of: swapping the top routing labelin the first label stack with a new routing label when the first labelstack contains more than two routing labels.
 7. The method as defined inclaim 6, wherein the second wavelength corresponds to a second opticalfrequency, the second optical frequency being mapped to a second binaryrepresentation, the second binary representation being divided into asecond plurality of fields, at least one of the second plurality offields corresponding to a routing label in a second label stack, a toprouting label in the second label stack indicating a third network node.8. The method as defined in claim 7, wherein the top routing label inthe second label stack corresponds to the new routing label.
 9. Themethod as defined in claim 8, wherein the network accommodates 2^(N)frequencies in the form of,f _(i) =f ₀ +i·Δf wherein i=0, 1, . . . 2^(N)−1, wherein the secondoptical frequency is defined by,f _(i) _(out) =f _(i) _(in) +2^(N−L)(l−l ¹)Δf and,i_(out) =i _(in)+2^(N−L)(l−l ¹) wherein fi_(in) represents the firstoptical frequency, l¹ represents the value of the top routing label inthe first label stack, l represents the value of the new routing label,and L represents the bit length of the top routing label in the firstlabel stack.
 10. The method as defined in claim 1, further comprisingthe step of: pushing a new routing label onto the first label stack. 11.The method as defined in claim 10, wherein the second wavelengthcorresponds to a second optical frequency, the second optical frequencybeing mapped to a second binary representation, the second binaryrepresentation being divided into a second plurality of fields, at leastone of the second plurality of fields corresponding to a routing labelin a second label stack, a top routing label in the second label stackindicating a third network node.
 12. The method as defined in claim 11,wherein the top routing label in the second label stack corresponds tothe new routing label.
 13. The method as defined in claim 12, whereinthe network accommodates 2^(N) frequencies in the form of,f _(i) =f ₀ +i·Δf wherein i=0, 1, . . . 2^(N)−1, wherein the secondoptical frequency is defined by,$f_{i_{out}} = {f_{0} + {{\lfloor \frac{( {f_{i_{in}} - f_{0}} )}{\Delta\; f} \rfloor \cdot 2^{- L} \cdot \Delta}\; f} + {{2^{N - L} \cdot l \cdot \Delta}\; f}}$and,$i_{out} = {\lfloor \frac{i_{in}}{2^{L}} \rfloor + {2^{N - L} \cdot l}}$wherein fi_(in) represents the first optical frequency, l represents thevalue of the top routing label in the second label stack, and Lrepresents the bit length of the top routing label in the second labelstack.
 14. The method as defined in claim 1, wherein the firstwavelength is the different from the second wavelength.
 15. The methodas defined in claim 1, wherein the first wavelength is the same as thesecond wavelength.
 16. The method as defined in claim 1, wherein atleast another one of the first plurality of fields corresponds to atermination field indicating an end of the first label stack.
 17. Themethod as defined in claim 1, wherein at least another one of the firstplurality of fields corresponds to a contention field fordifferentiating the first wavelength from a third wavelength.
 18. Themethod as defined in claim 17, wherein the data is a first data, whereinsecond data is received at the first network node via a third opticalsignal having the third wavelength, and wherein the first optical signaland the third optical signal have similar routing paths through thenetwork.
 19. An apparatus for routing data within an optical networkhaving a plurality of network nodes, the apparatus comprising: anoptical receiver for receiving data at a first network node via a firstoptical signal having a first wavelength, the first wavelengthcorresponding to a first optical frequency, the first optical frequencybeing mapped to a first binary representation, the first binaryrepresentation being divided into a first plurality of fields, at leastone of the first plurality of fields corresponding to a routing label ina first label stack, a top routing label in the first label stackindicating a second network node; and an optical transmitter fortransmitting, based at least partially upon the top routing label, thedata from the first network node to the second network node via a secondoptical signal having a second wavelength.
 20. The apparatus as definedin claim 19, wherein the first wavelength is the different from thesecond wavelength.
 21. The apparatus as defined in claim 19, wherein thefirst wavelength is the same as the second wavelength.
 22. The apparatusas defined in claim 19, wherein at least another one of the firstplurality of fields corresponds to a termination field indicating an endof the first label stack.
 23. The apparatus as defined in claim 19,wherein at least another one of the first plurality of fieldscorresponds to a contention field for differentiating the firstwavelength from a third wavelength.
 24. The apparatus as defined inclaim 23, wherein the data is a first data, wherein second data isreceived at the first network node via a third optical signal having thethird wavelength, and wherein the first optical signal and the thirdoptical signal have similar routing paths through the network.